1. Field of the Invention
The present invention relates to the sorting of particle mixtures according to particle composition. In particular, this invention relates to the use of vibrational analysis to differentiate among particles of varying composition. The term "particle" is used throughout this specification to denote any single discrete element in a mixture, regardless of size.
2. Description of the Prior Art
Vibrational analysis is known to be useful for the rapid automated sorting of particles in a moving stream. Systems utilizing this technique generally involve directing a stream of particles, one at a time, against a strike plate, and analyzing the mechanical vibrations occurring in the strike plate as a result of the impact. Differences in one or more characteristics of the vibrations are then related to differences in the particle size or composition. The deflection of certain particles from the stream on the basis of these vibrational characteristics is then done by automatic signal processing.
A wide range of particle properties can be used as a basis for the differentiation. Examples are hardness, density and elasticity. Deflection to isolate the unwanted particle may be achieved by mechanical, pneumatic, magnetic or electrical means, depending on the nature of the particle.
The concept of sorting through vibration analysis has been applied to a wide variety of mixtures ranging from pulverized refuse to bulk food, and it is conceivably applicable to particles ranging in size from granular to relatively large dimensions. The technique is useful for either sorting particles into portions having certain properties in preselected ranges, or for checking for and removing substandard units from a production line. The food nut industry has disclosed the technique as potentially useful for separating nutmeats from shell fragments after the whole nuts have been cracked and broken into pieces. See for instance, Parker et al., U.S. Pat. No. 4,212,398, July 15, 1980. Limitations of throughput, range and sensitivity, however, have shown the technique to be impractical for on-line sorting in the walnut industry.
All of the various systems developed to date employ a single impact plate. Vibrations resulting from the impacts in such systems have multiple frequency components, and different types of particles tend to overlap substantially in their range of response. The overlap makes selection difficult and creates a high degree of inaccuracy. A further problem with existing systems is the need for separating the particles into a single file stream aimed at the strike plate so that the impacts can be analyzed individually. This either slows down the process considerably or, if a large number of parallel analyzers is used, requires sufficient equipment to break the flow into an equal number of single file streams. Finally, single file sorting often requires that the particles be accelerated. This causes product damage and increases the amount of waste produced.